1. Field of the Invention
The present invention relates to the field of memory devices, and in particular, to memory devices with the ability to store multiple logical states.
2. Description of the Background
Semiconductor memory today is based on a binary system. In each cell of a binary system memory, only two logical states can be stored, namely, "1" or "0". If the number of logical state in a memory cell can be increased without increasing the area of the cell, then the memory density can be increased. Since the semiconductor memory constitutes the most widely used product in integrated circuits today, increasing the memory density is highly desirable.
FIGS. 1a and 1b show the folding electrical characteristics, i.e. having positive (R1 and R3) and negative (-R2 and -R4) resistance sections, and a circuit diagram, respectively, of a 2-peak resonant tunneling diode (RTD). FIGS. 2a and 2b show the folding characteristics and a circuit diagram of a multiple-peak RTD. Here, each triangle of the circuit diagram corresponds to a current peak. Such RTDs have been used in ternary logic systems and memory cells as disclosed in U.S. Pat. No. 4,853,753 by Capasso et al, Potter et al (1990 International Symposium on Circuits and Systems, Digest of Papers pp. 2557-2561), and U.S. Pat. No. 4,907,045 by Ando. In Capasso et al, a memory cell is disclosed employing RTDs with multiple peak folding characteristics where stable operating points are defined where a load line of a resistor external to the cell intersects the positive resistance sections of the folding characteristics. Such cells require careful processing as a change in resistance can limit and vary the position of the stable operating points, these points being used to define multiple logic states.